1. Field of the Invention:
This invention relates to a clamping head assembly for underpinning of joints between two mitred members. It was specially devised for the underpinning of picture frames but may have other uses.
2. Description of the Prior Art
Prior clamping head arrangements have required the exact positioning of a pair of mitred members in a right-angled locator to be done by hand, following which the members are clamped mechanically in position by the clamping head. In a first type of clamping head, this has been done by the application of downward pressure only. In a second type, a right-angled clamping member lying in the general plane of the mitred members is moved into a clamping position along the line of the mitre.
It is a difficult, skilled operation to cut the mitres exactly and to ensure exact positioning of the mitred members before clamping. When any inaccuracy occurs, the prior clamping heads do not correct this and they may accentuate misalignments or distortions, which are then preserved in the underpinned joint.
For example, in the first described type of prior clamping head, if the mitres are not cut exactly vertical, the downward pressure of the clamping head may open up the front face of the mitre. In a picture frame for example, this is the face which is visible in use. If the mitred ends are not correctly positioned or are released before clamping takes place, the second type of clamping head may force them apart, since the force exerted along the direction of the mitre has a component in the direction across the frame members, tending to open the joint.
With the second-mentioned prior type of clamping head, the working table of the machine is obstructed by the right-angled clamping member, making it difficult to manipulate the mitred members and slowing down the underpinning operation.